Exploring High Performance Polymers for Protective Coatings

Transcription

1 Exploring High Performance Polymers for Protective Coatings By Hitesh Soni, PhD, Manager Epoxies, Emerald CVC Thermoset Specialties and Andrew Recker, Technical Director Additives, Emerald Specialties When most of us think about coatings, the architectural coatings used to protect and decorate interiors and exteriors generally come first to mind. These types of paints must stand up to UV, weather conditions, the occasional food and beverage spills, children s artwork, and the requisite household cleaning products. Challenging, yes; however, another important class of coating EM and Special Purpose Coatings must stand up to some of the most demanding conditions (chemical resistance, like acids, alkali, solvents, salt, UV) in industrial, container, marine, and oil/gas applications. In fact, the total cost of corrosion on the U.S. economy has been estimated at $300 billion annually. 1 The cost of corrosion control has been valued at $121 billion, and organic protective coatings is by far the largest preventive measure employed. 1 Critical functions of protective coatings include avoiding accidents related to structural failure, contamination of materials contained in the equipment, and protecting the environment from a chemical release. These types of coatings categories also provide value by extending service life, protecting the investment of capital equipment, or avoiding use of higher cost construction materials. Unlike architectural coatings that are typically applied in pleasant weather, protective coatings are often field-applied in adverse conditions. For example, the pipelines used to transport chemicals, oil, or gas across long distances must be painted in place, possibly in several different types of extreme environments through which they pass on their journey to operations and customers including very hot or cold, arid, or high moisture climates and they must be able to transport materials safely and without incident in each of these field environments. To meet these needs, the solutions in the formulator s toolkit differ in a number of aspects from architectural coatings. To provide the necessary protection, coatings for EM and special purpose applications are often applied in multiple coats, such as a primer, basecoat and in some cases, a topcoat (Figure 1). The final system, depending on the stresses inherent to the end use, must resist chemicals, UV light/chalking, impact, cracking, chips, and moisture. It must also keep corrosion at bay an electrochemical process precipitated when steel reacts with oxygen and water to form rust and meet other common performance criteria such as adhesion, flexibility, and ability to FIGURE 1 Cross section of recommended coatings systems depending on needed level of protection. Topcoat (150µm) Interlayer (200µm) 7% Primer (250µm) SUBSTRATE 6% 15% 14% Epoxy 18% PU, PU/Acrylic, Silicone Epoxy, PU/Acrylic Zinc with Zinc Epoxy Thicker application of a typical three-layer protective coatings system, particularly for the zinc-rich primer, will provide an excellent and longer lasting balance of properties for highly corrosive environments, typically lasting about 15 years. FIGURE 2 Global consumption of high performance corrosion coatings by resin type (includes container and offshore coatings). 2 40% Topcoat ( µm) Interlayer ( µm) Primer (50-80µm) Urethane Alkyd Acrylic SUBSTRATE Inorganic Zinc ther (Silicone, Vinyl, etc.) cure, which becomes easily complicated by factors such as cold or moisture. As a result of these performance needs, the main variables which impact the warranty and expected life of the protective coating are thickness and the composition of each protective layer comprising a coatings system. The first critical choice the formulator must make is the type of polymer resin, the backbone of the formulation, often categorized by the type of binder used: epoxies are the leading polymer used, followed by polyurethanes, acrylics, alkyds, and others such as specialty silicones, all of which are often used in conjunction with zinc primers Reprinted from the Journal of Coatings Technology with permission from the American Coatings Association. PU, PU/Acrylic, Silicone Epoxy, PU/Acrylic Zinc with Zinc Epoxy Thinner application of a typical three-layer protective coatings system will provide a very good balance of properties for a mildly corrosive environment, typically lasting 0 15 years. PAINT.RG 49

2 (Figure 2). (Zinc is used as a measure against corrosion protection, since the zinc will oxidize sacrificially before the substrate.) This article will focus on the characteristics of the major polymers and how they may be used to optimize performance in protective coatings. VERVIEW F PLYMERS Polymer resins are the high-molecular-weight compounds that are the starting component of any coating formulation and will necessarily guide the ultimate properties. Within the five major classes of polymers, there are numerous variations of resin technology and modifications, including both waterborne and solventborne types. However, unlike architectural coatings where waterborne predominates, the use of solventborne, exempt solvent and 100% solids systems are the most common protective solutions, as these enable the use of more durable high-t g resins. The general performance characteristics of available technologies also tend to follow the general guidelines of their parent polymer category (Table 1). EPXY Epoxies are indisputably the workhorse among protective coating polymer resins, valued for their ability to provide an excellent balance of characteristics needed in a wide span of demanding protective coatings applications: excellent adhesion and resistance to corrosion, chemicals, hydrocarbons, and acid. They are also easy to handle and apply consistently. Some types can even be applied in water-immersed environments. Typically, epoxies are used in primers and as an interlayer coating, paired with a zinc basecoat and a topcoat to provide gloss and prevent discoloration and chalking. Epoxies are created by reacting phenols with acetone or formaldehyde and epichlorohydrin, resulting in a diglycidyl ether. However, there is a wide range of epoxy chemistries available, and unsurprisingly, these diverse structures produce different properties and benefits. While 1K epoxy systems are used in powder coatings applications (heat cured), the significantly greater usage for epoxy coatings is 2K liquid coatings. Liquid epoxies include bisphenol A, bisphenol F and novolac types, which are cured using an amine-based agent at low temperature, room temperature, or with some heat (up to 60 C), depending upon type. Certain modified epoxies have been optimized to improve cure speeds and provide better curing at lower temperatures, such as modified bisphenol A types, which may be cured at temperatures down to 5 C. ther benefits of modified epoxies can include improved chemical, hydrocarbon, and acid resistance. Epoxies cured using a cycloaliphatic amine can also provide a range of improved properties; they are more light-stable than other epoxies and also have better resistance to heat, abrasion, and impact. From a structural standpoint, the polarity, molecular weight, and number of epoxy linkages per molecule drive many of the epoxy s properties, impacting flexibility, cure properties, resistance to solvents, hydrocarbons and acid, and suitability for special applications, such as food contact. Impact resistance/strength and abrasion resistance are also important considerations, and epoxies such as 1K fusion-bonded epoxies are used in extreme environments such as hydraulic fracturing operations, where very high temperatures TABLE 1 Relative Performance of Protective Coatings by Polymer Type PRPERTY PLYURETHANE (ALIPHATIC) ACRYLATES PLYESTER/ VINYL ESTER ALKYD EPXIES RESRCINL AND PHENL NVLACS a LER (BIS A) CYCLALIPHATIC EPXY ADHESIN GD GD GD GD EXCELLENT EXCELLENT EXCELLENT WATER RESISTANCE GD PR GD GD VERY GD EXCELLENT VERY GD CURE SPEED EXCELLENT EXCELLENT EXCELLENT GD VERY GD GD PR SUSCEPTIBILITY T MISTURE DURING CURE CNSIDERABLE CNSIDERABLE CNSIDERABLE NT A FACTR NT A FACTR NT A FACTR NT A FACTR GLSS RETENTIN EXCELLENT VERY GD PR-GD b VERY GD PR PR GD ABRASIN RESISTANCE EXCELLENT GD GD PR GD GD GD UV RESISTANCE GD EXCELLENT PR-GD b GD PR PR GD CHEMICAL RESISTANCE GD PR GD PR EXCELLENT GD GD HYDRCARBN RESISTANCE GD PR PR PR EXCELLENT GD GD ACID RESISTANCE PR PR GD PR EXCELLENT GD PR EASY T USE NEEDS SPECIAL HANDLING ISCYANATE, SAFETY HANDLE WITH CARE (a) Includes standard bisphenol F epoxies (b) Based on cycloaliphatic polyester system YES SMELL, PRPER PPE NEEDS SPECIAL HANDLING STYRENE, PRPER PPE YES YES YES YES PRPER PPE PRPER PPE PRPER PPE PRPER PPE 50 MARCH 2016

3 FIGURE 3 Molecular structures of epoxy chemistries. and pressures necessitate paramount performance for these characteristics. Figure 3 shows the common types of epoxy molecules. For high performance industrial maintenance applications, where resistance to fluids and corrosion is a concern, use of epoxies with reactive modifiers increases crosslink density, which in turn maximizes chemical resistance. The choice of the right reactive modifier, whether a multi-functional reactive monomer based on glycidyl ether/ ester and/or a reactive liquid polymer such as a carboxy-terminated butadiene acrylonitrile, can optimize corrosion resistance while also overcoming issues with flexibility or brittleness. In addition to the choice of resin and reactive modifier, amines may be used in the formulation not only to increase cure speed but also to increase crosslink density, which in turn impacts the final properties of the coatings. The formulation considerations are vast and will be the subject of a future Formulator s Corner article. While corrosion and acid resistance is significantly improved with greater crosslinking and rigidity, as shown in the example in Figure 4, increasing hydrocarbon resistance of the coating is largely related to the coating s repellency, which increases as the polarity decreases. As a result, lower-molecular-weight bisphenol F/A epoxies are preferred for applications where hydrocarbon resistance is critical. However, formulators are seeking alternatives to these for indirect/direct food contact applications due to health and safety concerns. PLYURETHANE Most polyurethane coatings are 100% 2K systems created through the reaction of Cycloaliphatic Diglycidyl Ester LER - 24 Hour H 2 S 4 Immersion H 3 C CH 3 Standard Bisphenol A Epoxy Epoxy Phenol Novolac Epoxy n FIGURE 4 Corrosion resistance of a standard liquid bisphenol A epoxy resin (left) vs a low viscosity phenol novolac epoxy resin (right). EPALLY Hour H 2 S 4 Immersion polyol and isocyanate, resulting in a urethane, though there are some specialty 1K and even 3K systems available. Urethanes are widely used in protective coatings due to their excellent gloss, color retention, and resistance to mechanical stressors. Various versions also have excellent chemical resistance. Some formulators may prefer a lower-cost acrylic topcoat for lower-duty applications, which also provides good color retention but to a lesser degree (polyurethanes will generally not yellow for 10+ years). However, polyurethanes provide better flexibility, abrasion and scratch resistance, toughness over varying temperature cycles, and acid etch resistance than acrylics. In general, polyurethanes offer a good balance of properties, but those properties may vary widely depending on the final formulation, given that urethanes are highly tailorable. H 2 C HC H 2 C CH 3 C CH 3 Cycloaliphatic Hydrogenated Bisphenol A Epoxy Resin H Standard Bisphenol F Epoxy H Resorcinol Novolac Epoxy CH 2 CH CH 2 EPALLY Day H 2 S 4 Immersion Polyurethanes are very often used as topcoats, where they can provide an attractive, high-gloss finish and resistance to chemicals and mechanical factors, with some caveats related to their relative complexity, cost, and uneven film formation compared to a less-tough acrylic topcoat. Most commonly, they are used in outdoor architectural structures, bridges and structural steel, and newer 1K moisture-cured, catalyzed polyurethanes permit application to damp surfaces and can reduce preparation costs. Recent developments also include solventless urethanes, which have become a common selection for water/wastewater, storage tanks, and oil and gas. ACRYLIC Acrylics are available in latex, solventborne (lacquer or enamel), and powder PAINT.RG 51

4 types. While waterborne, latex acrylics are very common in architectural applications, acrylics use is limited in many protective coatings due to their susceptibility to water, acids, hydrocarbons, and chemicals, as well as limitations related to the use of high T g resins, which can negatively impact the durability of many acrylic coatings. The high odor of solventborne types also presents concerns for application. However, acrylics overall are an attractive cost-effective option generally as a topcoat for formulators looking to satisfy needs for fast curing, gloss, adhesion, and UV stability in environments where corrosion, chemicals, hydrocarbons, and acids are not significant wear factors. Their lower cost and comparable aesthetics relative to polyurethanes also make them popular for light- to medium-duty industrial environments and commercial structures, including bridges, cargo containers, outdoor applications, and other low-corrosive, non-immersive end uses. Where VC restrictions and health and safety considerations are a concern, waterborne acrylics have also been used to replace solventborne alkyds. Lowsolids, solventborne acrylic lacquers may also sometimes be used as a replacement for high-vc chlorinated rubber. Highsolids solventborne acrylics, or enamels, are negligibly used in protective coatings due to their lower stability. ALKYD Alkyds are complex oil-modified polyesters, derived from the polymerization between an alcohol, such as glycerol, and a dicarboxylic acid or its anhydride, such as phthalic anhydride. The selected reactants determine the desired properties. Adding an unsaturated oil to the ester causes branching. When exposed to air upon application, they provide properties such as flexibility and adhesion. The amount of oil will classify the alkyd as short-oil (<40%), medium-oil (40-60%) or long-oil (>60%), with greater oil content leading to longer curing times and greater flexibility, as well as improved wetting for better adhesion. Many alkyds are used in applications such as topcoats over a higher-performing primer and in fire-retardant primers. They provide good weatherability, color, and gloss in mild atmospheres but tend to crack with age due to continued oxidation and crosslinking, and their generally low resistance to water, chemicals, and corrosion makes them inappropriate for immersive or high-moisture environments. They are also a good choice for high temperatures, maintaining adhesion, flexibility, and toughness on steel substrates up to temperatures around 107 C, though high temperatures are likely to cause discoloration. As is often the case, solventborne alkyds tend to outperform their waterborne counterparts with respect to gloss, corrosion protection, and hardness, but waterborne alkyds have increased in use for low-vc applications. Today s modified versions of waterborne alkyds bridge that performance gap somewhat, reducing the damaging effect of hydrolysis which shortens shelf life after reduction in water by using an acrylic shell around an alkyd core. Solventborne alkyds may also be modified to increase performance characteristics such as resistance to water, chemicals, and weather, typically using a silicone, vinyl, or urethane compound. SILICNE Silicones, commonly called polysiloxanes, have key coatings applications relating directly to the fundamental chemistry of the dimethyl Si linkage. The chemistry of the polymer provides extended UV resistance due to higher energy bonds and no conjugation. The performance increase, with regard to color and gloss retention for exterior applications, is by far the best available option. In a gloss retention over time test, the gloss retention after 2800 hours was 80% for polysiloxane, versus 30% for a polyurethane and under 20% for alkyds, acrylic, and standard bisphenol A epoxy systems. 3 The polysiloxane topcoat is considered a premium substitute for the standard acrylic-polyurethane topcoat paired with a zinc primer and epoxy intermediate base. This performance has caveats related to the relatively high raw material costs and the inability to recoat, which means that the coating must be removed using sandblasting before re-applying. Further work with the polysiloxane topcoat chemistry has provided hybrid technologies in epoxy/amine functional polysiloxanes attempting to eliminate the epoxy intermediate by providing better barrier properties while maintaining the high gloss and color retention. Another property derived from the chemistry of the polysiloxanes is the low energy, or low polarity, created from the methyl groups. The hydrophobicity produced by the polysiloxanes makes it difficult for objects to adhere. An example application is antifouling coatings, which prevent aquatic life from growing on the hull of ships, which would otherwise cause drag as the ship moves through the water and increase fuel costs, and could also result in the harmful translocation of invasive marine species. In the past, antifouling coatings would include a biocide such as copper or tin, which would then end up in the ecosystem and continue to kill organics. Today, some states are moving to ban copper and tin in coatings, and polysiloxane coatings are a popular choice that allows for the release of the aquatic life or fouling without toxicity to the environment. The focus of development for antifouling coatings is to improve the polysiloxane low energy property to the extent that lower and lower agitation provides the desired release until static release is achieved. Adhesion promoters or coupling agents are also an important application for siloxane chemistry. Hydroxyl functional silanol molecules can provide crosslinking through condensation with other hydroxyl functional binders, extenders, or even substrates. The increase in adhesion can be utilized for decreasing delamination issues and/or increasing corrosion resistance. EMERGING TRENDS As the protective coatings market continues to grow, so too does the diversity of chemistries available, and some formulators have looked to incorporate biorenewable materials. Biorenewables are based on biological feedstocks such as sugars or alcohols and are typically associated with a lower carbon footprint of the coating from cradle to grave, as well as eliminating some of the potential health and safety concerns of solvent-based coatings. For example, 52 MARCH 2016

5 an ERISYS modifier based on sorbitol is available as a multifunctional aliphatic resin and has shown potential for replacement for bisphenol A, a suspect endocrine disruptor. A bio-based alternative for the epichlorohydrin used in epoxies is also emerging. ther new chemistries are paradigm shifting technologies that are beginning to gain traction with continued development and study. For example, new innovations in nanotechnology such as multi-walled carbon nanotubes have been used effectively to increase corrosion resistance, UV stability, and other properties in epoxy formulations. Formulators may also be looking for alternatives to certain traditional technologies such as zinc primers, which are effective corrosion inhibitors but have generated some environmental concerns due to their toxicity to marine organisms, as well as rising costs due to a decreasing global supply. Alternatives to zinc include anticorrosive pigments such as zinc-free orthophosphates, polysulphates, and silicas. CNCLUSINS With overall upward trends leading to the growth of the protective coatings market, many new chemistries and innovations continue to work toward a holy grail coating that can offer the ultimate balance of toughness, flexibility, weatherability, resistance to corrosion, and less environmental impact. Meanwhile, the five umbrella polymer classes discussed here can provide some general insight as to the performance benefits and caveats that are typical to each and guide the formulator toward which chemistries may be right for a given application. Every end use will present its own blend of challenges to overcome, and thus far, no polymer is one size fits all. Epoxies continue to be the most pervasive chemistry for a good balance of properties. They are often paired with a cosmetic polyurethane or acrylic topcoat, which may also be used alone in lower-corrosive environments, and polysiloxanes, alkyds, and other chemistries should not be overlooked for applications where they are known to excel. Designing a coating formulation must be completed from the ground up, and just as you do not buy the furniture for a room before constructing the walls, the formulator must select the best polymer chemistry or package of chemistries before moving onto additional components. After this crucial step has been completed, the formulator can then move onto fulfilling the remaining performance needs of the coating using additives and other materials, which will be guided by the choice of polymer. ACKNWLEDGMENTS Thank you to Jenna Blankenship, Jeremy Pasatta, and Julie Vaughn of Emerald Performance Materials for additional contributions to the article. REFERENCES 1. Corrosion Cost and Preventive Strategies in the United States, A U.S. Department of Transportation Study Publication No. FHWA-RD , performed by Corrosion Control Technologies in cooperation with FHWA and NACE International, Fink, U., Linak, E., Kishi, A., and Ma, X., High Performance Anticorrosion Coatings. IHS Specialty Coatings , 104, McMillan, C., Red Means Go, Polysiloxane Technology on the Roosevelt Island Bridge. International Paint, LLC. Houston, TX PAINT.RG 53